Process of debundling carbon fiber tow and molding compositions containing such fibers

ABSTRACT

A process for debundling a carbon fiber tow into dispersed chopped carbon fibers suitable for usage in molding composition formulations is provided. A carbon fiber tow is fed into a die having fluid flow openings, through which a fluid impinges upon the side of the tow to expand the tow cross sectional area. The expanded cross sectional area tow extends from the die into the path of a conventional fiber chopping apparatus to form chopped carbon fibers, or through contacting tines of a mechanical debundler. Through adjustment of the relative position of fluid flow openings relative to a die bore through which fiber tow passes, the nature of the fluid impinging on the tow, the shape of the bore, in combinations thereof, an improved chopped carbon fiber dispersion is achieved. The chopped carbon fiber obtained is then available to be dispersed in molding composition formulations prior to formulation cure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. non-provisionalapplication Ser. No. 14/398,673, filed Nov. 3, 2014, that in turn is aU.S. National Phase Application of International Application No.PCT/US13/39041, filed 1 May 2013, that in turn claims priority benefitof U.S. Provisional Patent Application Ser. No. 61/641,136, filed May 1,2012, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention in general relates to a process for debundlingcarbon fiber tow and in particular, to a process for producing choppedand disbursed carbon fibers amenable to inclusion molding compositions.

BACKGROUND OF THE INVENTION

The use of fiber inclusions to strengthen a matrix is well known to theart. Well established mechanisms for the strengthening include slowingand elongating the path of crack propagation through the matrix, as wellas energy distribution associated with pulling a fiber free from thesurrounding matrix material. In the context of sheet molding composition(SMC) formulations and bulk molding composition (BMC) formulations;hereafter referred to collectively as “molding compositions”, fiberstrengthening has traditionally involved usage of chopped glass fibers.There is a growing appreciation in the field of molding compositionsthat replacing in part, or all of the glass fiber in moldingcompositions with carbon fiber. However, this effort has met withlimited success owing to differences between glass and carbon fibers.Specifically, these differences include fiber diameter with glass fibersused in molding compositions having typical diameters of between 16 and30 microns while carbon fibers typically have diameters of between 2 and10 microns. Additionally, whereas glass roving fabrics, or bundlestypically have tens to hundreds of individual fibers, carbon fiber towstypically come in bundles of thousands and even tens of thousands ofindividual fibers. A still further difference exists in the fiber-fiberinteractions where glass fibers tend to scatter and debundle uponchopping, Van der waals bonding and other interfiber surfaceinteractions tend to make carbon fiber disinclined from debundling afterchopping into desired lengths for use as reinforcement in a moldingcomposition. While the debundling of carbon fiber tows is addressed inlaboratory scale moldings through manual manipulation, problems existfor production scale debundling of carbon fiber tow into separatechopped carbon fibers.

Thus, there exists a need for a process to debundle carbon fiber towinto separated chopped carbon fibers in a continuous manner. Therefurther exists a need to facilitate interaction of carbon fibers withmolding composition components to enhance the strength of a resultingSMC or BMC

SUMMARY OF THE INVENTION

An improved device and method for debundling a large number of carbonfibers collectively forming a tow into dispersed chopped carbon fiberssuitable for usage in molding composition formulations is provided.According to an embodiment of the present invention, a carbon fiber towis fed into a die having fluid flow openings, through which a fluidimpinges upon the side of the tow to expand the tow cross sectionalarea. The expanded cross sectional area tow extends from the die intothe path of a conventional fiber chopping apparatus to form choppedcarbon fibers. Through adjustment of the relative position of fluid flowopenings relative to a die bore through which fiber tow passes, thenature of the fluid impinging on the tow, the shape of the bore, incombinations thereof, an improved chopped carbon fiber dispersion isachieved, compared to existing processes. The chopped carbon fiberobtained according to the present invention is then available in certainembodiments to be dispersed in molding composition formulations prior toformulation cure. Through control of the molding composition monomerpolarity, still further dispersion and anisotropy of the chopped carbonfibers is obtained.

A mechanical debundler that accepts tow like material, such as carbonfiber tow, as input in a top feed area between two opposing rollers withtines, where the first roller spins clockwise and the second rollerspins counter clockwise so as to pull the tow inward towards a lowertined roller that rotates faster than the upper two rollers, and as thetow goes through it gets pulled open and kinked to expand the tow toabout 5 times in height. In embodiments, the mechanical debundler mayalso be used simultaneously with heating, air blowing, or plasma. In theembodiment where plasma is used, the tines are charged to create plasmain region that tow is being teased open.

In an alternative embodiment of the present invention, a conventionallychopped fiber tow is dispersed in a polar monomer containing moldingcomposition.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1A is a transverse cross sectional view of a die according to thepresent invention operative in debundling carbon fiber tow;

FIG. 1B is a longitudinal cross sectional view of the die shown in FIG.1A; and

FIG. 2 is a second embodiment of a die operative in the presentinvention for debundling carbon fiber tow.

FIG. 3 illustrates a mechanical debundler according to an embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has utility in debundling a large number of carbonfibers collectively forming a tow into dispersed chopped carbon fiberssuitable for usage in molding composition formulations. According to thepresent invention, a carbon fiber tow is fed into a die having fluidflow openings, through which a fluid impinges upon the side of the towto expand the tow cross sectional area. The expanded cross sectionalarea tow extends from the die into the path of a conventional fiberchopping apparatus to form chopped carbon fibers. Through adjustment ofthe relative position of fluid flow openings relative to a die borethrough which fiber tow passes, the nature of the fluid impinging on thetow, the shape of the bore, in combinations thereof, an improved choppedcarbon fiber dispersion is achieved, compared to existing processes. Thechopped carbon fiber obtained according to the present invention is thenavailable in certain embodiments to be dispersed in molding compositionformulations prior to formulation cure. Through control of the moldingcomposition monomer polarity, still further dispersion and anisotropy ofthe chopped carbon fibers is obtained.

In an alternative embodiment of the present invention, a conventionallychopped fiber tow is dispersed in a polar monomer containing moldingcomposition.

Referring now to FIGS. 1A and 1B, an inventive die is shown generally at10. The die 10 has a bore 12 that is dimensionally larger than the crosssectional area of the carbon fiber tow T passed there through. At leasttwo fluid flow openings 14, 16, and 18 as shown in FIGS. 1A and 1B areprovided to allow fluid communication into the bore 12 and onto the sideof the carbon fiber tow T. The bore 12 is depicted having a triangularcross section. It is appreciated that the bore of an inventive die hasother cross sectional shapes such as circular, oval, hourglass-shape,and other polygonal cross sectional shapes. The fluid openings 14, 16,and 18 allow for a fluid to be injected into the bore 12 so as to impactthe side of the carbon fiber tow T and induce debundling so as toincrease the cross sectional area of the bundle T. According to thepresent invention at least two fluid openings are provided. It isappreciated that the fluid ingress need only occur through the fluidopenings as the fluid is able to leave the die 10 from the terminal dieface 20 that is proximal to a fiber chopping apparatus A. Through thecontrol of feed rate of the tow T and the operational speed of thechopper A, a controlled length debundled chopped fiber C is obtained.

As fluid impingement the fluid openings 14, 16, and 18 each intersectwith the bore 12 at an angle, alpha (α), beta (β), and gamma (γ),respectively. Each of these angles is independently variable. Inspecific embodiments, the angles alpha, beta, and gamma are eachindependently between 30 and 150 degrees, although it should beappreciated that other angles are operative herein.

Without intending to be bound to a particular theory, it is believedthat, the fluid enters the bore 12 under conditions such that the fluidpenetrates into the tow T creating a larger void volume in the resultantexpanded cross sectional area bundle. Fluid entering the bore 12 that asan expanding fluid tends to travel along a path of least resistance, theconditions are preferably established that favor fluid penetrationthrough the tow T thereby increasing the cross section of the tow Tprior to the fluid exiting through a lower pressure fluid flow openingthan the one through which the fluid entered alone or in combinationwith exiting through the terminal die face 20. As best shown in FIG. 1B,it should be appreciated that fluid flow openings need not all exist ina single plane where fluid flow outlet 18 is downstream from openings 14and 16 relative to the movement of the tow T.

Another embodiment of an inventive die is shown in FIG. 2 generally at30 with a circular bore 12′ and fluid openings 14′ and 16′;

As used herein, the term “fluid” is intended to include gasses, liquids,and aerosol atomized fluids. Fluids operative herein for debundlingcarbon fiber tow illustratively include air, nitrogen, noble gasses,carbon dioxide, carbon monoxide, and steam. It is appreciated thatorganic molecules, and silanes, above the respective boiling temperatureof each are also operative herein as gaseous fluids used to debundlecarbon fiber filaments. Liquid fluids suitable for debundling carbonfiber tow illustratively include high polarity liquids with a relativepolarity of greater than about 0.5. In addition to a simple fluid, afluid according to the present invention also carries an additive suchas particulate, radicals, coupling agents, and combinations thereof.Particulate suitable for entrainment within a fluid impinging upon acarbon fiber tow illustratively include carbon black, calcium carbonate,colloidal silica, titanium dioxide, and combinations thereof. Couplingagents operative herein illustratively include epoxies,organo-titanates, organo-zirconates, hydroxyl methyl resorcinol, andcombinations thereof. Radicals operative as additive; especially gaseousfluids illustratively include ozone, singlet oxygen, and plasma. It isappreciated that control of factors such as cross sectional area of thebore, the shape of the bore, inlet pressure of fluid, and relativeposition of fluid exit openings affect the degree of carbon fiber towdebundling, to produce dispersed carbon fiber strands after beingchopped from the tow by a chopping apparatus A is readily achieved.

As shown in FIG. 2 where like numerals correspond to the meaningsascribed thereto with respect to FIGS. 1A and 1B, another die as part ofan inventive process is provided. Without intending to be bound to aparticular theory, simultaneously impinging upon a carbon fiber tow fromtwo fluid flow openings as shown in FIG. 2, under conditions that limitthe twisting of the carbon fiber tow is suitable in inventiveembodiments for successful debundling.

FIG. 3 illustrates an inventive mechanical debundler 40 that accepts towlike material, such as carbon fiber tow, as input in a top feed areabetween two opposing rollers with tines 48, where the first roller 42spins clockwise and the second roller 44 spins counter clockwise so asto pull the tow inward towards a lower tined roller 46 that rotatesfaster than the upper two rollers (42, 44), and as the tow goes throughit gets pulled open and kinked to expand the tow to about 5 times inheight. In embodiments, the mechanical debundler may also be usedsimultaneously with heating, air blowing, or plasma to separate the tow.In certain embodiments, heat is applied that is sufficient to remove anysizing or other conventional surface coatings on the surface of thecarbon fibers. In still other embodiments heat is applied under an inertor reducing atmosphere to promote pyrolysis of the sizing from the corecarbon fibers. A plasma is readily generated with a conventional plasmagenerator source to treat the tow fibers prior to, during, or subsequentto engagement with the second roller 44. In the embodiment where plasmais used, the tines are charged to create plasma in the region that thetow is being teased open. It is appreciated that a tow is contacted witha mechanical debundler in concert with air debundling or as a standalone process.

An inventive process after a carbon fiber tow has been fed through a diewith impingement of fluid onto the side of the tow and penetrating thecarbon fiber tow so as to increase the cross sectional area, or throughthe mechanical debundler, the tow is then chopped into preselectedlengths of carbon fiber strands. The resultant strands are thendispersed in an SMC, BMC or RTM formulation for subsequent molding andcure. It has been found that such chopped fiber strands tend to disperseand achieve a greater degree of both fiber debundling and anisotropywhen the molding composition is more polar. In specific embodiments ofthe present invention, the chopped carbon fibers so produced aredispersed in a methyl methacrylate monomer. Other suitable monomers fromwhich a molding composition formulation is produced illustrativelyinclude unsaturated polyesters, epoxies, and combinations thereof. Amolding composition formulation based on epoxy illustratively includesbis-phenol-A and Novolac based epoxy terminated resins. Suitable curingagents for such an epoxy based molding composition formulationillustratively include anhydrides such as trimellitic anhydride, methyltetrahydrophthalic anhydride (MTHPA), nadic methyl anhydride (NMA), di-and tri-functional amines, and combinations thereof.

An alternative embodiment of the present invention involves dispersingconventional chopped and bundled carbon tow in a molding compositionmonomer or solution containing monomer with a relative polarity ofgreater than 0.26, and in certain embodiments greater than 0.5, and instill other embodiments between 0.5 and 0.8. Relative polarity isdefined per Christian Recihardt, Solvents and Solvent Effects in OrganicChemistry, Wiley-VCH, 3^(rd) edition, 2003.

The chopped carbon fibers produced according to an inventive process arereadily dispersed in molding composition formulations prior to cure as asubstitute for, or in combination with glass fibers. As a result ofdebundling carbon fiber tow according to an inventive process, areinforced SMC, BMC or resin transfer molding (RTM) cured article isformed that has a lower density overall, and a lower percentage byweight loading of fibers. Additionally, through the use of couplingagents superior tensile strength is achieved. Additionally, it is ofnote that the inventive process as a continuous process for producingchopped carbon fibers is amenable to usage with production scalemanufacture.

The resulting chopped and debundled carbon fibers find particularutility in an SMC having an inner portion containing from 10 to 40% byweight carbon fibers of the inner portion, with an outer skin of SMCbased on the commercially available TCA (Continental StructuralPlastics) containing glass fiber containing between 10 and 60% glassfiber by weight of the TCA portion, as embodied in U.S. Pat. No.7,655,297. The ratio of thickness of the inner portion to the outer skinranges from 01-10:1. The resulting SMC inner portion and outer skinlayers are either cured separately and joined or cured in contact withone another. Such a dual layer SMC with an inner portion containingcarbon fibers is noted to have a density that is 10, 20, 30 and even 40%lower than the comparable article formed wholly from TCA. In this way alightweight article is formed that retains the high surface glass of aclass-A surface associated with TCA.

Patent documents and publications mentioned in the specification areindicative of the levels of those skilled in the art to which theinvention pertains. These documents and publications are incorporatedherein by reference to the same extent as if each individual document orpublication was specifically and individually incorporated herein byreference.

The foregoing description is illustrative of particular embodiments ofthe invention, but is not meant to be a limitation upon the practicethereof. The following claims, including all equivalents thereof, areintended to define the scope of the invention.

The invention claimed is:
 1. A process for debundling carbon fiber towinto chopped carbon fibers comprising: feeding the carbon fiber towdefined by a bundle cross sectional area and a tow surface into a diehaving a bore, the die having an entrance opening, at least two fluidflow openings and a separate exit opening in a terminal face of the die,where the bore is sized larger than the bundle cross sectional area withthe proviso that if the at least two fluid flow openings are twoopenings, that the two openings are not diametrically positioned througha circular cross sectional bore, and where the carbon fiber tow moveslinearly through the die from the entrance opening to the exit opening;impinging on the carbon fiber tow within the bore through a firstopening of the at least two fluid flow openings with a fluid at a flowrate to expand carbon fiber tow to an expanded cross sectional areagreater than the bundle cross sectional area within said bore; exitingthe expanded cross sectional area carbon fiber tow from the exit openingin the terminal die face of the die; and chopping the expanded crosssectional area carbon fiber tow upon exiting said die to form thechopped carbon fiber.
 2. The process of claim 1 wherein the carbon fibertow has at least 1,000 carbon fibers therein.
 3. The process of claim 1wherein chopping occurs proximal to the terminal face of said die. 4.The process of claim 1 wherein the bore is a cross sectional shape oftriangular, circular, oval, or polygonal.
 5. The process of claim 1wherein the at least two fluid flow openings are laterally displacedalong a length of the bore.
 6. The process of claim 1 wherein at leastone of the fluid flow openings defines a fluid exhaust.
 7. The processclaim 1 wherein the fluid is air.
 8. The process of claim 1 wherein thefluid is gaseous and further comprises particulate.
 9. The process ofclaim 1 wherein the fluid is gaseous and further comprises radicals. 10.The process of claim 1 wherein the fluid is gaseous and furthercomprises a coupling agent.
 11. The process of claim 1 furthercomprising dispersing the chopped carbon fiber in a molding compositionprepolymer formulation.
 12. The process of claim 11 wherein said moldingcomposition prepolymer comprises polar monomers.
 13. The process ofclaim 12 wherein said polar monomers comprise a majority by weight ofsaid molding composition absent fiber and fillers.
 14. The process ofclaim 1 further comprising contacting said tow with a plurality ofrotating tines.
 15. A process for debundling carbon fiber tow intochopped carbon fibers comprising: feeding the carbon fiber tow definedby a bundle cross sectional area and a tow surface into a die having abore, the die having at least two fluid flow openings, where the bore issized larger than the bundle cross sectional area with the proviso thatif the at least two fluid flow openings are two openings, that the twoopenings are not diametrically positioned through a circular crosssectional bore; impinging on the carbon fiber tow through a firstopening of the at least two fluid flow openings with a fluid at a flowrate to expand carbon fiber tow to an expanded cross sectional areagreater than the bundle cross sectional area within said bore; andchopping the expanded cross sectional area carbon fiber tow upon exitingsaid die to form the chopped carbon fiber contacting said tow with aplurality of rotating tines while simultaneously exposing said tow to atleast one of heat or plasma.
 16. The process of claim 15 comprisingcontacting said tow with a plurality of rotating tines whilesimultaneously exposing said tow to at least one of heat or plasmawherein heat is applied under an inert or reducing environment.
 17. Aprocess for debundling carbon fiber tow into chopped carbon fiberscomprising: feeding a continuous carbon fiber tow defined by a bundlecross sectional area and a tow surface into a mechanical debundlercomprising: a feed area that accepts said continuous tow between twoopposing rollers with tines, where a first roller of said two opposingrollers spins clockwise and a second roller of said two opposing rollersspins counter clockwise so as to pull the continuous tow inward towardsa single lower tined roller that rotates faster than the upper tworollers; expanding said continuous tow to about 5 times in loft.
 18. Theprocess of claim 17 further comprising chopping the expanded crosssectional area carbon fiber tow upon exiting said mechanical debundlerto form the chopped carbon fiber.
 19. The process of claim 17 furthercomprising applying at least one of blowing air, heat or plasma to saidtow in a region between said lower tined roller and the upper tworollers.